1. Field of the Invention (Technical Field)
The present invention relates to flexible borehole liners, including flexible borehole liners used to effectuate pore fluid sampling and other similar uses, and more particularly to the sealing capability of everting liners in subsurface boreholes.
2. Background Art
Flexible borehole liners are now in commercial use for sealing boreholes and providing isolation for discrete spatial resolution of ground water sampling. General descriptions of the technology may be found on the World Wide Web at www.flut.com. U.S. Pat. Nos. 5,176,207 and 5,725,055 also offer examples of borehole flexible liner methods and apparatuses. The interior pressure of a liner is maintained to be greater than the pore pressure in the medium adjacent to the borehole, thus forcing the liner fabric against the borehole wall to achieve a seal of the borehole against flow into and along the hole.
However, a concern remains with the function of the borehole liner as a sealing device for very long time periods. That concern stems from the molecular diffusion processes. As is well known, molecular diffusion allows the relatively slow transport of contaminants, in solution, through thin barriers from a high concentration volume into an adjacent lower concentration volume. A relevant diffusion process is discussed in U. S. Pat. No. 5,804,743.
In a like manner, tri-chloro-ethylene, and similar contaminants of ground water, can migrate via diffusion from the ground water external to a borehole, through a flexible liner material, and into the interior of the fluid-filled liner. Such diffusion may compromise or defeat the purpose of the liner, which is intended generally to seal the borehole against contamination migration. In time, the diffusion process may propagate the contaminant along the liner length to other locations interior to the fluid-filled liner. Thereafter, diffusion out of the liner into the surrounding medium may contaminate the pore fluids of the medium outside of the liner, remotely from the point(s) of diffusion into the liner interior. This particular diffusion transport path, i.e., 1) into the liner, 2) along the liner, and 3) out of the liner, can bypass the seal against contaminant transport otherwise expected to be achieved by the pressurized liner.